USE OF H-1 LONGITUDINAL RELAXATION-TIMES IN THE SOLUTION STRUCTURE OFPARAMAGNETIC PROTEINS, APPLICATION TO [4FE-4S] PROTEINS

The accuracy of the solution structures determined by NMR is often poo r around paramagnetic centers because the properties of the near proto ns are strongly perturbed by the electronic spin. The structural infor mation contained in the relaxation rates of these protons has been ext racted here by measuring the longitudinal relaxation times with the in version-recovery total correlation spectroscopy (IR-TOCSY) sequence ba sed on the recovery of cross peaks. In addition to measurements with n onselective inversion-recovery for nonoverlapping signals, reliable da ta have been obtained for a majority of main-chain protons from Chroma tium vinosum high-potential ferredoxin. When a small and constant cont ribution from diamagnetism as well as the electronic spin distribution over the [4Fe-4S] cluster are taken into account, the shortest longit udinal relaxation times depend directly on the distance separating the protons from the paramagnetic center. This indicates that electron-nu clei dipolar interactions are the most efficient relaxation mechanism for these protons. However, the expected dependence of the relaxation rates as the sixth power of the distance has to be corrected because o f induced relaxation among fast relaxing protons. This approach reveal s that the solution structure of the protein is significantly differen t from the crystal structure around Phe-48. In addition, it provides a n independent confirmation of the actual electronic structure of the [ 4Fe-4S](3+) cluster in the protein. The method devised in this work, w hich does not rely on specific enrichment, should be useful to improve the determination of NMR-derived solution structures of paramagnetic macromolecules.